According to the ideas current in the art, the production of glazed ceramic tiles is carried out by one or other of two general processes.
The first of the known production processes provides for forming the ceramic body of the tile in a traditional manner, drying it and then firing it to obtain the so-called biscuit-ware.
By ceramic bodies is here meant any bodies made with batches of natural raw materials such as clay, kaolin, feldspar, wollastonite, talc, calcium carbonate, dolomite etc., or synthetic materials like calcined kaolin (grog), pure oxides (allumina, silica, corundum), with the addition or not of binders to increase the raw mechanical strength.
Such bodies, undergo chemical and structural changes during a heating treatment and acquire substantially new mechanical properties, passing from the low toughness of the raw state to the hard, tough and brittle state after the heat-treatment.
Consequently, in this specification, for "firing of ceramic bodies", it generally means that heating treatment, which causes the above mentioned substantial alteration of the mechanical characteristics of the ceramic bodies by means of chemical reactions, crystalline modification, and components' melting.
The biscuit is then glazed and given a further firing to bring about in the glaze the structural change--strictly dependent on the nature of the components of the glaze--which for the sake of simplicity will hereafter be called vitrification. This term may not be strictly correct for all the glazes used in the ceramic tile-marking industry which, during firing, may melt almost completely or only to a small extent, in this latter case the phenomenon being closer to sinterization.
However, as the firing of glaze on ceramic materials is well-known in the art, such linguistic simplification cannot lead to misunderstanding. By glazed there is here meant any composition adapted to develop under heating a vitreous, transparent or opaque surface, which can be glossy or dull, either in a wholly fritted from, i.e. melted previously, or raw, or in the form of differently composed mixtures.
This process of double-firing, first of the ceramic body and then of the glaze, has advantages but also disadvantages. In the first place, the first firing, the ensuing cooling, and then the second firing to vitrify the glaze represent a process that consumes much thermal energy and much time and calls for apparatus volumes which are large for the volumes of tiles produced.
It might be thought that in double-firing the first firing to produce the biscuit-ware can, if conducted in a workmanlike manner, produce biscuit-ware of optimal characteristics as a result solely of the nature of the starting clay mix; the fact is, however, that the requirements deriving from the subsequent glazing in practice condition the preparation of the biscuit-ware.
The glazes are generally prepared by wet milling, so as to contain for example from 30% to 60% of water.
If dry glazes are employed, a wet treatment is then needed to guarantee adherence to the ceramic body.
When the glazes are applied to biscuit-ware of porous structure, i.e. not vitrified, the plentiful presence of water is not a source of difficulties; for in such conditions the biscuit-ware readily absorbs the water and the glaze therefore adheres well to the body.
But when the biscuit-ware is vitrified, special treatments are necessary before the firing proper of the glaze, in particular a pre-heating to obtain rapid evaporation of the water; nevertheless, if it is wished to obtain an acceptable adherence of the glaze, such treatments prove highly critical and thus represent considerable difficulties as far as the process in question is concerned.
In consequence, the problems involved in glazing vitrified ceramic bodies defeat the apparent freedom to form the ceramic body in an independent manner as in double-firing, since such independent formation will only at the cost of serious complications permit the formation of tiles with vitrified ceramic body having desirable mechanical strength properties.
Because of the problems entailed in the double-firing process, a more recent proposal suggests firing both the clay mix and the glaze in a single operating step--the so called single-firing process. According to this process the raw ceramic bodies are dried, pre-heated and glazed when they are sufficiently hot to allow an evaporation of the water contained in the glaze. However, the pre-heating does not prevent a certain amount of water from being absorbed by the raw clay mix--and moreover in an uneven manner, so that in practice absorption proves greater at the edges of the article since the edges cool more rapidly.
This phenomenon can lead to a fissuring of the article at its edges, thus reducing its strength.
Another intrinsic difficulty of the single-firing process is that it calls for somewhat complex movement of the raw tiles, which have to be glazed in the said condition and the delicacy of which is known.
In the single-firing process the raw glazed tile is then fired in a single cycle up to a temperature that fires both the body mix and the glaze (vitrification of the glaze).
As it requires only one furnace or kiln and only one firing operation, single-firing is clearly an important advance on traditional double-firing: the apparatus can be less extensive, and the thermal energy dispersed between the firing of biscuit-ware and the firing of the glaze is saved.
If the reactions occuring during the firing are sufficiently known, with single-firing it is also possible to obtain nearly all the effects typical of double-firing, although it may appear to be a limitation to have to provide a clay mix firing temperature compatible with the characteristics of the glaze that vitrifies during the same firing, with consequent loss of the operational independence peculiar to double-firing.
However, the greatest difficulty in single-firing is the predisposition of the gases released during the firing of the ceramic body to vitiate the evenness and compactness of the glaze.
The importance of this phenomenon is readily seen if it is recalled that a tile weighing 1 kg formed from a mix containing 10% of calcium carbonate and brought to a temperature of 1000.degree. C. releases a volume of carbon dioxide of about 90 liters.
The gas released from the ceramic body passes through the glaze, in which remain small bubbles which leave it to a certain extent in a porous condition.
Thus, though single-firing obviates the glaze-firing in the case of vitrified ceramic bodies, the former leads to glazes which, even though they have excellent adherence, are of a relatively porous consistency.
It has also been suggested to apply an insulating layer, known as engobe, between glaze and ceramic body in the single-firing process in order to reduce the possibility of interaction between them during firing; but the application of engobe diminishes the satisfactory adherence of the glaze that is characteristic of single-firing.
In short, double-firing gives rise to porous-ceramic body tiles, which thus have lower mechanical strength but an adherent and highly compact glaze, whereas single-firing gives rise to tiles with vitrified ceramic body of high mechanical strength and with a glaze that is satisfactorily adherent, but relatively porous.